pcre2compat man page

This document describes the differences in the ways that PCRE2 and Perl handle regular expressions. The differences described here are with respect to Perl versions 5.26, but as both Perl and PCRE2 are continually changing, the information may sometimes be out of date.

1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does have are given in the pcre2unicode page.

2. Like Perl, PCRE2 allows repeat quantifiers on parenthesized assertions, but they do not mean what you might think. For example, (?!a){3} does not assert that the next three characters are not "a". It just asserts that the next character is not "a" three times (in principle: PCRE2 optimizes this to run the assertion just once). Perl allows some repeat quantifiers on other assertions, for example, \b* (but not \b{3}), but these do not seem to have any use.

3. Capturing subpatterns that occur inside negative lookaround assertions are counted, but their entries in the offsets vector are set only when a negative assertion is a condition that has a matching branch (that is, the condition is false).

4. The following Perl escape sequences are not supported: \l, \u, \L, \U, and \N when followed by a character name or Unicode value. (\N on its own, matching a non-newline character, is supported.) In fact these are implemented by Perl's general string-handling and are not part of its pattern matching engine. If any of these are encountered by PCRE2, an error is generated by default. However, if the PCRE2_ALT_BSUX option is set, \U and \u are interpreted as ECMAScript interprets them.

5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2 is built with Unicode support (the default). The properties that can be tested with \p and \P are limited to the general category properties such as Lu and Nd, script names such as Greek or Han, and the derived properties Any and L&. PCRE2 does support the Cs (surrogate) property, which Perl does not; the Perl documentation says "Because Perl hides the need for the user to understand the internal representation of Unicode characters, there is no need to implement the somewhat messy concept of surrogates."

6. PCRE2 does support the \Q...\E escape for quoting substrings. Characters in between are treated as literals. This is slightly different from Perl in that $ and @ are also handled as literals inside the quotes. In Perl, they cause variable interpolation (but of course PCRE2 does not have variables). Note the following examples:

The \Q...\E sequence is recognized both inside and outside character classes.

7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code}) constructions. However, there is support PCRE2's "callout" feature, which allows an external function to be called during pattern matching. See the pcre2callout documentation for details.

8. Subroutine calls (whether recursive or not) were treated as atomic groups up to PCRE2 release 10.23, but from release 10.30 this changed, and backtracking into subroutine calls is now supported, as in Perl.

9. If any of the backtracking control verbs are used in a subpattern that is called as a subroutine (whether or not recursively), their effect is confined to that subpattern; it does not extend to the surrounding pattern. This is not always the case in Perl. In particular, if (*THEN) is present in a group that is called as a subroutine, its action is limited to that group, even if the group does not contain any | characters. Note that such subpatterns are processed as anchored at the point where they are tested.

10. If a pattern contains more than one backtracking control verb, the first one that is backtracked onto acts. For example, in the pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the same as PCRE2, but there are cases where it differs.

11. Most backtracking verbs in assertions have their normal actions. They are not confined to the assertion.

12. There are some differences that are concerned with the settings of captured strings when part of a pattern is repeated. For example, matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE2 it is set to "b".

13. PCRE2's handling of duplicate subpattern numbers and duplicate subpattern names is not as general as Perl's. This is a consequence of the fact the PCRE2 works internally just with numbers, using an external table to translate between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b>B), where the two capturing parentheses have the same number but different names, is not supported, and causes an error at compile time. If it were allowed, it would not be possible to distinguish which parentheses matched, because both names map to capturing subpattern number 1. To avoid this confusing situation, an error is given at compile time.

14. Perl used to recognize comments in some places that PCRE2 does not, for example, between the ( and ? at the start of a subpattern. If the /x modifier is set, Perl allowed white space between ( and ? though the latest Perls give an error (for a while it was just deprecated). There may still be some cases where Perl behaves differently.

15. Perl, when in warning mode, gives warnings for character classes such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE2 has no warning features, so it gives an error in these cases because they are almost certainly user mistakes.

16. In PCRE2, the upper/lower case character properties Lu and Ll are not affected when case-independent matching is specified. For example, \p{Lu} always matches an upper case letter. I think Perl has changed in this respect; in the release at the time of writing (5.24), \p{Lu} and \p{Ll} match all letters, regardless of case, when case independence is specified.

17. PCRE2 provides some extensions to the Perl regular expression facilities. Perl 5.10 includes new features that are not in earlier versions of Perl, some of which (such as named parentheses) were in PCRE2 for some time before. This list is with respect to Perl 5.26:

(a) Although lookbehind assertions in PCRE2 must match fixed length strings, each alternative branch of a lookbehind assertion can match a different length of string. Perl requires them all to have the same length.

(b) From PCRE2 10.23, back references to groups of fixed length are supported in lookbehinds, provided that there is no possibility of referencing a non-unique number or name. Perl does not support backreferences in lookbehinds.

(c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $ meta-character matches only at the very end of the string.

(d) A backslash followed by a letter with no special meaning is faulted. (Perl can be made to issue a warning.)

(e) If PCRE2_UNGREEDY is set, the greediness of the repetition quantifiers is inverted, that is, by default they are not greedy, but if followed by a question mark they are.

(f) PCRE2_ANCHORED can be used at matching time to force a pattern to be tried only at the first matching position in the subject string.

(g) The PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY and PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.

(h) The \R escape sequence can be restricted to match only CR, LF, or CRLF by the PCRE2_BSR_ANYCRLF option.

(i) The callout facility is PCRE2-specific. Perl supports codeblocks and variable interpolation, but not general hooks on every match.

(j) The partial matching facility is PCRE2-specific.

(k) The alternative matching function (pcre2_dfa_match() matches in a different way and is not Perl-compatible.

(l) PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT) at the start of a pattern that set overall options that cannot be changed within the pattern.

19. Perl has different limits than PCRE2. See the pcre2limit documentation for details. Perl went with 5.10 from recursion to iteration keeping the intermediate matches on the heap, which is ~10% slower but does not fall into any stack-overflow limit. PCRE2 made a similar change at release 10.30, and also has many build-time and run-time customizable limits.